Microelectronic devices are fabricated on semiconductor wafers using a variety of techniques, e.g. including deposition techniques (CVD, PECVD, PVD, etc) and removal techniques (e.g. chemical etching, CMP, etc). Semiconductor e.g. silicon wafers may be further treated in ways that alter their mass e.g. by cleaning, ion implantation, lithography and the like. These treatment techniques typically cause a change in the mass at or on the surface of the semiconductor wafer. The configuration of the changes to the surface are often vital to the functioning of the device, so it is desirable for quality control purposes to assess wafers during production in order to determine whether they have the correct configuration.
A number of existing measurement techniques are known. For etching treatment, one known technique is to break a treated wafer and perform a detailed analysis of its cross-section. If the analysis shows treatment to have been successful, it is assumed that the batch of wafers manufactured at the same time as the tested (broken) wafer also have the correct configuration. The disadvantage of this process is that the broken wafers cannot be used and are therefore wasted, and that the process is typically interrupted (i.e. fabrication stopped) every time a wafer from a batch is tested. Continuous production is therefore not feasible.
Other known measurement techniques depend on the type of treatment or the properties of materials created by the treatment. For example, treated wafers can be measured using ellipsometry when they contain dielectrics or wafers can be tested using resistivity probes when conductive metals are deposited thereon.
WO 02/03449 discloses an apparatus and method for investigating semiconductor wafers in which changes in the mass of the wafer are determined to assess various properties of the wafer, e.g. enable fabrication of the wafer to be monitored. A common method of obtaining mass measurements is to use a very sensitive force sensor to measure the force (weight) due to gravity. At medium levels of accuracy this force can be assumed to be due solely to the mass of the wafer. However, if higher levels of accuracy are needed, other forces may need to be taken into account.
One such force (disclosed in WO 02/03449) is caused by atmospheric buoyancy. In semiconductor metrology, a semiconductor wafer is usually measured in an atmosphere (i.e. not a vacuum). The wafer therefore displaces a volume of this atmosphere, which causes an up thrust force. The up thrust force depends on the atmospheric (air) density, which in turn depends on numerous factors including temperature, atmospheric pressure, relative humidity and air composition. The up thrust force reduces the apparent weight of the wafer detected by the force sensor.
WO 02/03449 discloses a method of compensating or correcting for the effect of atmospheric buoyancy. Sensors are provided to monitor temperature, pressure and relative humidity. A processor receives measurements from these sensors and uses them to calculate the air density, which can be used to compensate for buoyancy in a corresponding weight measurement. The processor can calculate buoyancy from the calculated air density, together with the weight measurement and density information about the wafer.